Avebury has a river to the southwest, the Wintebourne and Marden also has the Avon.
IF there were ashes scattered into the river at mid winter (and there is NO evidence yet) then some of the rivers would have been frozen most of the time.
At Durrington the ashes would have passed Stonehenge quickly whereas they could just as easily have been taken overland.
What about the Access to the henges by boat?
For what its worth I think these places were more orientated to the mid winter sunset.
It would only be a few days after solstice that you noticed there was more light in the day not less darkness at night.